Chlorine is supposed to take care of most of the microbes floating around in pools, but human waste, it seems, is stubbornly resistant to being sanitized.

That’s the conclusion of a group of researchers from the Centers for Disease Control (CDC), who collected water samples from 161 filters in public and private swimming pools, as well as water parks in Atlanta last summer. What they found trapped in those filters was enough to make swimmers think twice before logging their laps. More than half of the samples were contaminated with E. coli, which the investigators say comes from one primary source — swimmers pooping in the pool.

The study, published in the latest Morbidity and Mortality Weekly Report, specifically looked at pools in Atlanta, but the researchers say such contamination is likely a widespread problem in U.S. pools, thanks to swimmers not washing themselves off before taking a dip. According to the scientists, each of us carries about 0.14 grams of fecal material into the pool — and that doesn’t include accidents or cases of diarrhea. Among municipal pools, the genetic testing for pathogens detected E. coli in 70% of the filters, while 66% of the water parks contained the bacteria and 49% of pools in private clubs showed evidence of the contamination.

“These findings indicate the need for swimmers to help prevent introduction of pathogens, e.g., taking a pre-swim shower and not swimming when ill with diarrhea, [for] aquatics staff to maintain disinfectant level and pH according to public health standards to inactivate pathogens, and state and local environmental health specialists to enforce such standards,” the authors write in their report.

When a pool is properly chlorinated, however, bacteria like E. coli should be killed off, since proper pH levels typically take care of the issue. According to the CDC, it takes less than a minute for E. coli to be inactivated if chlorine levels are adequate, about 16 minutes to control Hepatitis A virus, about 45 minutes to kill off the Giardia parasite and over 10 days for a Crypto parasite.

But just one diarrhea accident can cause an infection for anyone who gets a mouth full of pool water. Fortunately, the testing did not reveal strains of E. coli 0157, a particularly virulent form of the bacteria that was responsible for several outbreaks, and deaths, from serious foodborne illnesses.

According to NPR, because the researchers only analyzed the samples for genetic signatures of different pathogens, they couldn’t determine whether the bugs were alive, and potentially reproducing in the water, but there were no pool-related diseases reported in Atlanta during the summer the samples were gathered.

Thomas Lachocki, the CEO of the National Swimming Pool Foundation, says that in order to be properly chlorinated, pools should contain 1-4 parts per million of chlorine and pH levels should be within 7.2–7.8. “You can go to any mass market store and go into the pool chemical aisle and buy test stripes. All of these have chlorine and pH tests. In five seconds, you can do a quick analysis yourself and have an idea of what the various levels are,” he says.

But if you don’t have the time to do your own testing, look for clear water. “You should always be able to see the bottom of the pool clearly. Usually if the water is cloudy, something with the filter or chemicals isn’t right,” says Lachocki. “Clear water doesn’t mean everything is alright, but cloudy water is an absolute positive sign that something is not right.”

Here are some additional recommendations from the CDC for ensuring a bug-free dip:

Don’t swim when you have diarrhea.

Shower with soap before you start swimming.

Take a shower to rinse off before you get back into the water.

Take bathroom breaks every 60 minutes.

Wash your hands after using the toilet or changing diapers.

Try not to swallow the pool water.

If you have young children:

Take children on bathroom breaks every half-hour to hour or check diapers frequently.

Change diapers in the bathroom or diaper-changing area and not at poolside where pathogens can rinse into the water

There may be no way to completely sanitize a pool, but the latest analysis of what could be lurking in the water should motivate lifeguards and pool managers to be more vigilant about testing those waters more frequently. People should outnumber the pathogens in any pool.

We are so clean, but sickly because we kill most bacteria around us that support life. Chlorine kills everything, and not every bacterium is bad for us. And besides, I never heard of anyone getting sick from bacteria infection after swimming in a chlorinated swimming pool; not lakes. So much for that.

Have any illnesses been attributed to this, or is this more "OMG! Bacteria!" fluff? Guess what, there's bacteria everywhere! There's billions of them on and in you right now! Even E Coli! Yes you, you have e coli on you right now. The question to be asked is not so much if there is bacteria in the water (there ALWAYS is), but at what concentrations. Bacteria can only cause illness if they are in high enough concentrations, otherwise, we'd all be dead from the millions of harmful bacteria we all carry around with us. Unless there is something wrong with the filtration system and/or the chemistry is off, swimming pool water is the cleanest water you can swim in. Swimming in fresh water lakes doesn't mean swimming in clean water. There are WAY higher concentrations of bacteria, as well as other pathogens, notably amoebas, in fresh water lake than in any properly maintained pool. As someone who has been a swimming pool professional for over a decade, it gets really, really old reading the same exact hysterical, ill informed garbage year after year after year. I've dealt with thousands of pools and customers and not once have I ever heard of someone getting sick from a swimming pool. Of course contamination can take place, but that is not the norm. Would you stop eating at all restaurants if you heard one of them had "bacteria"? Guess what, they all do too! It would be nice if people that wrote scientific articles had a clue as to what they are talking about, instead of just parroting some meaningless, out of context, unexplained statistics.

The good thing is that the CDC has come out with the Model Aquatic Health Code that recommends all swimming pools to install Secondary Disinfection System such as UV Sterilization or Corona Discharge Ozone. Treating pool water like they do sewage and reclaim water is gaining popularity in our pool industry. Our industry needs to be held accountable for the health and safety of our bathers. We need to protect ourselves and our children where we can and the proper treatment of pool water is a very big place to start.

Our city started sending out cards with the water bill (city owned utility) every May. They remind people not to drink the pool water, disclose how much fecal material pool water is likely to have, even with proper water treatment, and practically beg people not to visit the pool when they're sick. It's a mandatory 3-day closure if there's a fecal incident in the pool. Making sure people are aware of the situation ahead of time has helped keep our pools a lot cleaner than they used to be.

When I took an introductory microbiology course in college, the professor said "the whole world is covered with a layer of bacteria - just try to eat where it's thinnest. " In fact, there are bacteria all around us every day, and with a little common sense, they're no problem for most healthy individuals (of course, the very young, old, and immunosuppressed are a different story). So go ahead and swim, just don't swallow the water, and don't stress too much about it.

Remember: "they couldn’t determine whether the bugs were alive, and potentially reproducing in the water, but there were no pool-related diseases reported in Atlanta during the summer the samples were gathered."